The present invention relates generally to the production of glass sheets and, more particularly, to an improved method of and apparatus for precisely controlling the temperatures of heated glass sheets in a mass production operation.
One process that has been successful in producing bent, tempered sheets of glass, such as are commonly used in glazing closures for automobiles and the like, is the horizontal press bending technique. This technique generally includes heating pre-trimmed flat sheets of glass to their softening or bending temperatures by advancing them on a roll conveyor through a heating furnace, bending the heated sheets to a desired curvature or shape between a pair of complementary mold members and then tempering the same by chilling the bent sheets in a controlled manner to a temperature below the annealing range of glass.
It should be appreciated that the glazing closures formed by the above-described process must be bent to precisely defined shapes as dictated by the configuration and size of the sight openings and the overall styling of the vehicles in which the closures are to be installed. Moreover, the glazing closures must be properly tempered to increase their resistance to damage resulting from impact and, in the event of breakage, to fragment into relatively small harmless particles as opposed to the large, jagged, potentially dangerous pieces otherwise resulting from untempered glass sheets when broken. Additionally, the bent and tempered glazing closures must meet stringent optical requirements whereby they are free of surface defects and optical distortions that would interfere with clear vision therethrough.
Probably the single most significant factor in meeting all of the above-mentioned requirements resides in heating the sheets to an optimum temperature level during the heating phase to properly condition the glass sheets for further processing. If a heated sheet exits the heating furnace at a relatively cool temperature for example, it will not be sufficiently soft for expedient and proper bending. Moreover, it will not retain the necessary heat required for subsequent tempering. On the other hand, if the sheet leaving the furnace is overheated, it will be extremely pliable with attendant loss of deformation control and will tend to sag out of the desired shape beyond the close tolerances prescribed. Also, overheating tends to degrade the surface quality of the finished product as a result of heat stains, roll marking, pitting and the like. While the optimum temperature range to which the sheets must be heated for satisfactory further processing can be readily calculated, problems are encountered in consistently reaching this desired temperature level and maintaining a multiplicity of glass sheets within such range in a mass production operation. This is due to the inherent, although slight, temperature variations generated by the irregular heat output of the heating elements, whether gas fired or electrical resistance elements, within the furnace and from other extraneous sources which influence the temperature of the heating atmosphere. In any event, it has been found that the temperatures of successive sheets exiting the furnace, as monitored by sophisticated temperature measuring devices, varies frequently and sometimes from sheet to sheet.
Attempts have been made to solve this problem by varying the thermal input to the heating elements in accordance with glass temperature variations from a desired level. However, these attempts haven't been entirely satisfactory because of a lagging heat input response, i.e., a time delay before the adjusted thermal input is adequately reflected in the heating atmosphere and imparted to the advancing glass sheets. Other attempts involve manually adjusting the rate of conveyor speed to compensate for temperature variations. However, it is virtually impossible to manually effect the necessary adjustments accurately in a minimum of time because of human error and/or miscalculations, thus seriously impairing efficiency in a mass production operation. Moreover, the complete concentration and constant surveillance required of the operator contributes significantly to fatigue, further increasing the possibilities of human error and poor judgment.